Plate making sensitive materials are light-sensitive materials used in a photographic plate making step in the printing industry (as described in U.S. Pat. Nos. 3,600,174, 4,126,472, 3,847,518, etc.), such as for converting a light-shade image into a halftone image or photographing a line drawing, etc.
Generally, with respect to these plate making sensitive materials, silver images obtained by development processing are halftone images or line drawing images. When they are used for making printing plates, a treatment called reduction treatment is frequently carried. This treatment is performed in order to obtain delicate reproduction of tone or satisfy artistic expression of the images adapting to printing characteristics, by which fine correction of images is carried out partially or completely. The corrections may include reduction of the area of dots or widening or narrowing of the width of line drawing.
Therefore, aptitude for reduction treatment is a very important property in plate making sensitive materials.
When carrying out reduction treatment of plate making sensitive materials having halftone images or line drawing images formed by exposure and development, it has been known to use a method which comprises contacting metal silver forming the halftone images or line drawing images with a reducer.
For example, reducers using a reducing component such as permanganate, ferric salt, cerium (IV) salt, red prussiate, bichromate or persulfate, etc., have been described in Mees, The Theory of the Photographic Process, pages 738-739 (1954, published by Macmillan Co.).
Since the reduction treatment means the treatment comprising oxidizing silver images by a reducer to dissolve them, then image density is necessarily lowered by this treatment.
Therefore, a measure of the extent to which halftone image can be corrected can be represented by a reduction of the area of dots while maintaining the black density of each dot at a specified value or more.
In the present specification, the term "reduction width" means a reduction of the area of dots from the area of dots before the reduction treatment when the black density of the dots is reduced by the reduction treatment to the lowest value necessary for the photographic plate making step. Thus, the wider the reduction width is, the higher the reduction treatment aptitude is.
A method for improving reduction ability is described in Japanese Patent Application (OPI) No. 68419/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") wherein mercapto compounds, such as monothioglycerol, .alpha.-amino-ethanthiol, etc., are used for the reduction treatment. However, the reducer is specific and difficult to use, because its reduction rate is different from that of conventionally used reducers. Further, it is possible to widen the reduction width while increasing the covering power by softening the emulsion membrane, but the required film strength cannot be obtained by this method.
Another method for widening the reduction width is increasing the silver content. This method is effective because the ability to correct images by the reduction treatment is increased when there is a greater amount of silver in the silver images per unit area. However, since silver is very expensive and rare, increasing the amount of silver is not preferred with respect to the cost of the plate making sensitive materials and economy of resources.
Accordingly, it is an important subject in this field to produce plate making sensitive materials having the required properties while using silver in an amount as low as possible.